Download Lesson 8: Title: Resistance is not futile.

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LESSON 8: TITLE: RESISTANCE IS NOT FUTILE.
Apparatus needed: ?
Preliminaries: check homework
Lesson:
Other circuits: 1) 3 resistors in a triangle. Io = 800 mA, I3 = 200 mA. Find I1, I2. Vo=10V, V2=4V.
Find V1, V3. Circuit components are always either in series or parallel -- there are no
other possibilities (except in 3-D ??). Show how circuit can be redrawn into familiar rectangular
pattern.
2) Add a resistor at the top -- not connected to anything. What is the current through it? - zero
3) battery, light bulb, wire shorting out the bulb. Io = 2A. What is the current through the wire
and the bulb? DEMONSTRATE.
* Electricity always follows the path of least resistance. Like water in a pipe: one pipe
constricted, open up another tap --'all' the water flows through it. Or when you have a shower
upstairs and someone turns on the tap in the basement! AMMETER: very low resistance.
Energy must be deposited somewhere: possibilities: ammeter, wire, or power supply.
VOLTMETER: very high resistance.
Resistance
A device that uses electrical energy is called a load. Loads always have some resistance; and a voltage
drop across the, since V = E/Q. (If E, then V)
Resistance describes the difficulty that electrons have moving through a material. All materials have
resistance (except superconductors).
(explain collisions with atoms, effect of temperature).
Speed of electrons:
 Electrons move at about 1.5 x 10^8 m/s – this is the instantaneous velocity (in random directions) –
in between collisions with atoms.
 The drift velocity is the speed that electrons actually move through a wire. This is about 7 E-4 m/s !!
The time for an electron to traverse the one meter length of the wire is therefore about 12 hours. The
drift velocity of electrons is very small due to the carrier scattering with the atomic vibrations
("phonons") at room temperature. In fact, it is just this scattering behavior that is responsible for the
linear relation between electric field and current density in a conductor, or in more familiar terms,
Ohm's Law.
How is this like Formula 1 race cars in a traffic jam?
 Note that the electromagnetic field, and hence any voltage changes, associated with the electrical
current propagates down the wire at a speed close to the speed of light (3xE8 m/sec). That’s why
when you turn on a switch the lights come on right away (all of the electrons move right away).
What will happen to the current through this bulb when I increase the voltage? (a bulb is non-ohmic!
better to use something else.) The resistance depends on (1) cross sectional area, (2) temperature, (3) the
material used (=conductivity, or =resistivity), (4) length of conductor.
R = L/A
Example: Number 10 wire has a diameter of 2.59 mm. How many meters of number 10 aluminum wire
are needed to give a resistance of 1 ?  for aluminum = 2.8 E-8 m ***Why a different  from table??
Metal
silver
copper
aluminium
lead
manganin (alloy)
eureka (alloy)
steel (varies)
Resistivity /( m)
1.6 × 10-8
1.7 × 10-8
3.2 × 10-8
21.0 × 10-8
44.0 × 10-8
49.0 × 10-8
10 to 100 × 10-8
Material
carbon
graphite
germanium
silicon
pyrex glass
PTFE
quartz
Resistivity /( m)
35 to 5000 × 10-8
800 × 10-8
0.65
2.3 × 10-3
1012
1012 to × 1016
5 × 1016
Resistivity values at room temperature. For metals, resistivity increases as temperature increases. For
semiconductors and many insulators, the opposite is true. Any impurities (also alloys) increase
resistivity significantly.
Resistance is measured in Ohms, symbol  (omega)
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(Analog meters) How to measure resistance: 1. pick a scale, 2. connect probles together, 3. zero meter,
4. measure and record. * analog: mirror to avoid parallax error.
Resistor Colour Codes:
Three bands: 1, 2 = number (no decimal!). 3 = multiplier. 4 = tolerance.
(gold=5%, silver = 10%, none = 15%
0=black,
1=brown,
2=red,
3=orange
4=yellow
5=green
6=blue
7=violet
8=gray
9=white
Example: what is the resistance of a resistor with 4 bands: red, yellow, orange, silver?
Activity: measuring R
Ask student to calculate the resistance of a resistor, then measure it. (can also do in series and parallel)
(or measure the resistance of 5 different things). How do you tell if a light bulb/ fuse is blown?
resistance = . Same with motors (high or low?)
NOW:
either finish the LED lab or start the Ohm’s law lab.
Homework:
Evaluation: